(a) Field of the Invention
The present invention relates to a method for preparing a dialkyl carbonate. More particularly, the present invention relates to a method for preparing a dialkyl carbonate in which a dialkyl carbonate such as dimethyl carbonate is economically prepared in an environmentally friendly manner at a higher yield while reducing generation of a by-product.
(b) Description of the Related Art
Dimethyl carbonate (DMC) as a representative dialkyl carbonate material is colorless and odorless, and has an environmentally friendly molecular structure without toxicity to a human body. Further, since dimethyl carbonate has various chemical reactivities, a reactive group such as a methyl, methoxy, or methoxycarbonyl group may be introduced. Dimethyl carbonate may replace highly toxic and corrosive chemicals such as dimethyl sulfate or methyl halides by introducing the reactive group. In addition, dimethyl carbonate has high solubility and may therefore be used as an environmentally friendly solvent to replace a solvent such as chlorobenzene. Recently, dimethyl carbonate has been used as a substitute material for phosgene as a raw material of polycarbonate, an additive for improving an octane number for vehicles, or an electrolyte solution of a rechargeable battery.
Dimethyl carbonate has been typically prepared with an alcohol such as methanol, phosgene, and a highly concentrated sodium hydroxide solution. However, it is known that there are many problems in views of safety and the environment due to toxic phosgene and the highly concentrated sodium hydroxide solution.
Another method for preparing dimethyl carbonate is an EniChem process. The EniChem process is a method for preparing dimethyl carbonate by oxidizing carbon monoxide and methanol using a monovalent copper chloride catalyst with oxygen in the air. However, the EniChem process has problems in that toxic carbon monoxide is used as a raw material, a conversion rate is low, and a cost of energy used to purify and circulate unreacted methanol is high due to generation of water as a by-product. Further, there are problems in that, since the monovalent copper chloride catalyst is readily oxidized into divalent copper ions, catalytic activity is reduced, and supplementation of a reaction device against corrosion and handling of explosion are required. Moreover, there is a problem in that, due to the presence of a small amount of chloride ions in products, a refining cost is rapidly increased when dimethyl carbonate is used as an electrolyte solution.
Still another method for preparing dimethyl carbonate is an Ube process in which methanol is oxidized into nitrogen dioxide to prepare methyl nitrate, water is removed, methyl nitrate reacts with carbon monoxide in the presence of a platinum catalyst to prepare dimethyl carbonate, nitrogen oxide comes into contact with air to again be converted into nitrogen dioxide, and nitrogen dioxide is circulated. The Ube process has problems in that, although a cost of energy for separation and purification is relatively low, the use of the highly toxic and corrosive carbon monoxide and nitrogen oxide requires an anti-corrosion reaction device, an anti-explosion safety device, and a precise concentration controlling device, and there is a risk of leakage of reactants.
Yet another method for preparing dimethyl carbonate is a Texaco process in which ethylene oxide (or propylene oxide) and carbon dioxide are reacted with each other at high pressure in the presence of a catalyst to prepare ethylene carbonate (or propylene carbonate), and then prepare dimethyl carbonate and ethylene glycol (or propylene glycol) through an ester exchange reaction with methanol. The Texaco process does not use carbon monoxide and thus has excellent safety as compared to the EniChem process and the Ube process. However, since the process is performed at high temperature and pressure, there is a risk of explosion due to leakage of ethylene oxide used as a raw material. Further, although the ester exchange reaction is performed at a high temperature, a conversion rate is not high, and thus there is a problem in that a large amount of energy is used to separate and purify unreacted materials as well as dimethyl carbonate and ethylene glycol as products.
Recently, a method for preparing dimethyl carbonate by directly reacting urea and methanol in the presence of a catalyst has been actively studied. In this method, inexpensive urea is used as a raw material, and since water as a by-product is not produced, a ternary azeotropic mixture such as methanol-water-dimethyl carbonate is not produced, and separation and purification processes may be easily carried out. Further, ammonia produced as the by-product may be reacted with carbon dioxide to be converted into urea and reused, and thus it is possible to prepare dimethyl carbonate by a further environmentally-friendly process.
As described above, the known methods for preparing dimethyl carbonate using urea and methanol are as follows. (1) a method for reacting urea and methanol in the presence of a zinc acetate catalyst (S. Bowden and E. Buther, J. Chem. Soc. 1939, Vol. 78), and (2) a method for synthesizing dialkyl carbonate by reacting urea, a primary aliphatic alcohol such as methanol, an organic metal compound, and a catalyst of a phosphine-based organic material (Peter Ball, Heinz Fullmann, and Walter Heintz, “Carbonates and Polycarbonates from Urea and Alcohol”, Angew. Chem. Int. Ed. Engl. 1980, Vol. 19, No. 9, pp 718-720; WO 95/17369). However, in these methods, it is difficult to synthesize a dialkyl carbonate such as dimethyl carbonate at a sufficient yield.
Further, a (3) method for preparing a dialkyl carbonate by using a catalyst of an organotin-based compound and a high boiling point electron donor compound, such as a polyglycol ether compound, as a co-catalyst (J. Yong Ryu, U.S. Pat. No. 6,010,976) is known, and various process patents are known based on the method (3). However, the aforementioned method has a drawback in that, since the catalyst of the organotin-based compound is unstable to water, activity thereof is reduced by water included in a raw material as an impurity, and also has a problem of toxicity. Further, the polyglycol ether compound used as the co-catalyst may be decomposed or polymerized at high temperatures, and thus activity thereof as the co-catalyst may be reduced due to an occurrence of a change in viscosity or carbonization. Moreover, it is difficult to regenerate the catalyst and the co-catalyst, which may cause an environmental pollution.
Meanwhile, a method (4) for preparing dimethyl carbonate by using a catalyst in which transition metal oxides such as Zn, Pb, Mn, La, or Ce and alkali (earth) metal oxides such as K, Na, Cs, Li, Ca, or Mg are impregnated in alumina or silica, and directly reacting urea and methanol using a reactor or a distillation column is disclosed in U.S. Pat. No. 7,271,120 B2. In this method, the catalyst and reactants may be easily separated. However, a reaction temperature at which dimethyl carbonate is synthesized is much higher than a boiling point of methanol, it is necessary to maintain a vapor-liquid equilibrium state at high pressure, and if produced ammonia and dimethyl carbonate are not discharged, a reaction yield may be reduced. Moreover, by-products such as N-methylmethyl carbamate (N-MMC) or N,N-dimethylmethyl carbamate may be formed due to a side reaction between methyl carbamate (MC) as an intermediate product and dimethyl carbonate.
As described above, in the method for preparing dimethyl carbonate through reaction distillation, in order to improve the reaction yield and distillation efficiency of dimethyl carbonate at the reaction temperature that is higher than the boiling point of methanol and the high vapor pressure of methanol, it is necessary to maintain the temperature and the pressure at which the vapor-liquid equilibrium is obtained, discharge ammonia, and obtain a distillate. In this case, the obtained distillate is an azeotropic mixture of dimethyl carbonate and methanol, and the concentration of dimethyl carbonate as the product may be reduced due to the azeotropic mixture at high pressure, which reduces productivity. Further, in this preparation method, by-products such as N-MMC or N,N-dimethylmethyl carbamate formed by reaction of methyl carbamate (MC) as the intermediate product may be produced in a large amount due to high reactivity of synthesized dimethyl carbonate (Yoshio Ono, “Dimethyl carbonate for environmentally benign reaction”, Pure & Appl. Chem., 1996, Vol. 68, No. 2, pp 367-375).
In addition, in (5) U.S. Pat. No. 5,534,649, urea or alkyl carbamate and alkyl alcohol are reacted in the presence of a quaternary ammonium salt-based ionic liquid such as tetramethylammonium hydrogen carbonate methyl ester or tetramethylammonium carbamate and an organotin-based catalyst to prepare a dialkyl carbonate. However, there is a problem in that a maximum yield of a dimethyl carbonate is 4.13%, which is very low.
Because of the aforementioned problems, a method of preparing a dialkyl carbonate such as dimethyl carbonate having various industrial purposes in an environmentally friendly manner at a higher yield while reducing generation of a by-product is continuously required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.